The present invention relates to a screen printing apparatus and a screen printing method for printing cream solder, conductive paste or like paste on a substrate.
Conventionally, screen printing has been adopted for printing cream solder, conductive paste or like paste on substrates in a process of mounting electronic components on a substrate. This method comprises a process of setting a mask plate with pattern holes provided therein according to the printing portions and a process of printing paste on a substrate by squeezing same through the pattern holes of the mask plate.
As a squeezing method for such screen printing, a method using a closed type squeegee head is known. In this method, a squeegee head provided with a paste storage container having an opening at the bottom thereof is used. With the opening pressed against the mask plate, the paste in the paste storage container is pressurized so that the paste is pressed and filled into the pattern holes of the mask plate through the opening. And then, the squeegee head is slid on the mask plate, thereby filling the paste into each pattern hole in order.
In such a conventional screen printing method using a closed type squeegee head, there are problems as described in the following. In order to fill the paste into the pattern holes in good condition, it is necessary to apply an appropriate pressure to the paste being stored. That is, if the pressure is not high enough, a sufficient amount of paste will not be filled into the pattern holes. Also, if the pressure is too high, the paste will be forced out of the pattern holes on the mask plate surface. Accordingly, the pressure applied should be appropriate to satisfy various conditions with respect to the type of the mask plate and the characteristics of the paste. However, it is not easy to properly set and maintain at all times the pressure and other printing conditions in accordance with printing objects which come in a variety of types and shapes. Therefore, it has been practically difficult to assure reliable printing qualities in such conventional printing.
The present invention is intended to provide a screen printing apparatus and a screen printing method by which reliable printing qualities may be obtained at all times.
A screen printing apparatus of the present invention for printing paste on a substrate comprises:
(a) a positioning member on which the substrate is placed,
(b) a mask plate with a pattern hole, in which the mask plate is located above the positioning member, and
(c) a squeegee head which slides on the surface of the mask plate;
wherein the squeegee head comprises:
(d) a printing section, and (e) a pressure section; and the printing section comprises:
(f) a storage section to store paste,
(g) a printing space portion with a bottom opening, in which the printing space portion is formed below the storage section,
(h) a transfer passage providing a communication between the storage section and the printing space portion, in which the transfer passage is formed between the storage section and the printing space portion, and
(i) scraping members disposed at the bottom of the printing space portion; and
the printing space portion includes (j) rolling guide surfaces to enhance the rolling motion of the paste,
wherein the paste stored in the storage section is pressurized by the pressure member while the squeegee head is sliding on the mask plate,
the paste stored in the storage section is filled into the printing space portion through the transfer passage, and
the rolling guide surfaces of the printing space portion serve to enhance the rolling motion of the paste, then the rolling paste is forced out of the bottom opening of the printing space portion and is printed on the substrate via the mask plate.
A screen printing method of the present invention for printing paste on a substrate comprises the steps of:
(a) sliding a squeegee head on the surface of a mask plate placed on the substrate,
(b) filling paste into pattern holes formed in the mask plate when the paste is forced out of a printing aperture at the bottom of the squeegee head according to the sliding operation of the squeegee head, and
(c) printing the paste, filled into the pattern holes, on the substrate;
wherein the step of sliding the squeegee head and forcing the paste out of the printing aperture comprises the steps of:
(d) pressurizing the paste stored in a storage section,
(e) forcing the pressurized paste out into a printing space portion through a transfer passage,
(f) enhancing the rolling motion of the paste forced out into the printing space portion, and
(g) forcing the rolling paste out of the printing aperture.
With this configuration, the paste is precisely printed on the substrate. The paste is accurately printed on the substrate in accordance with the shape of the pattern formed in the mask plate. As a result, it is possible to assure reliable printing qualities.